A number of methods and apparatus have been developed for obtaining a measurement of mean fibre diameter and fibre diameter distribution in a sample containing; a plurality of wool fibres having different diameters.
Two classes of instruments for the measurement of mean fibre diameter are:
1. Those which give an estimate of average diameter only.
2. Those which also give the distribution of fibre diameters within a sample including statistical information such as the variance of the diameters of the sample fibres.
In recent times the information given by the distribution of wool fibre diameter has come to be accepted as being required in some circumstances.
To accurately estimate the distribution of the fibre diameters a large number of measurements have to be made.
A particular method for measuring mean fibre diameter and fibre diameter distribution involves the measurements of fibre diameters in an optical microscope using a calibrated graticule to gauge the fibre diameters. This method is slow, tedious and prone to errors. These errors can arise from a number of sources including the optics, the conditioning of the fibres, and the judgment of the operators. Measurement of a few thousand fibres using this technique takes many hours to complete.
An instrument for determining fibre diameter distribution has been proposed by Lynch and Michie, Australian Patent No. 472,862 entitled "Optical Shadowing Method and Apparatus for Fibre Diameter Measurement".
In the apparatus described in 472,862 a light beam traverses a transparent measurement cell and falls on a photoelectric sensor.
Fibres dispersed and suspended in a clear liquid are caused to flow through the measurement cell and intercept the light beam. The reduction in the detected light intensity as a result of a fibre properly occluding the light beam is a function of the diameter of the fibre.
The apparatus includes a split photodetector and a processor to reject readings when a fibre end falls within the light beam. Ensuring that the amplitude of the signal from the two detecting elements of the split photodetector differed by less than 10%, was thought to be sufficient for acceptance of the measurement.
Instruments manufactured according to the teaching of the Lynch and Michie patent have been available for many years and are used to measure the diameter distribution of wool and other fibres.
Over this period a number of deficiencies, some of which are related to the validity of the individual fibre measurements, have become apparent.
Firstly, equal light occlusion on the two halves of the split detector is not sufficient to guarantee the validity or otherwise of a measurement. For example it has been observed that the diameter of some fibres varies more than 30% in less length than the beam diameter. These fibres can give an unequal response from the two halves of the split detector circuitry which would in turn reject the measurement, even though the measurement should have been accepted. Alternatively, measurements of fibres that have not fully crossed the light beam have been observed to have been accepted when they should have been rejected.
Secondly, the Lynch and Michie proposal assumed that the fibre snippets would be so dilute in the carrier liquid that the probability of two fibres being in the light beam at the same time would be negligible.
In practice, it has been found that for a typical measurement rate of 100 fibres per second the proportion of occurrence where two fibres appear in the light beam at the same time is significant. This effect appears in the diameter distribution graph as a second hump at double the value of the real distribution hump. The second hump has a significant affect on the second moment statistic, the variance.
The Lynch and Michie patent did not teach a method for rejecting the signals representing the occurrence of two fibres measured simultaneously in the light beam, but practical realisation of the instrument has included an apparatus whereby signal responses with a double peak were interpreted as representing two fibres in the beam simultaneously and were therefore rejected.
Observations have shown that the double peak detector does not pick up all multiple fibre events.